moisture transport and intraseasonal variability in …...the south american monsoon system (sams)...
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Moisture transport and intraseasonal variability in the SouthAmerica monsoon system
Leila M. V. Carvalho • Ana E. Silva •
Charles Jones • Brant Liebmann •
Pedro L. Silva Dias • Humberto R. Rocha
Received: 1 October 2009 / Accepted: 22 March 2010 / Published online: 14 April 2010
� The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract This paper examines moisture transport on
intraseasonal timescales over the continent and over the
South Atlantic convergence zone (SACZ) during the South
America (SA) summer monsoon. Combined Empirical
Orthogonal Function analysis (EOFc) of Global Precipita-
tion Climatology Project pentad precipitation, specific
humidity, air temperature, zonal and meridional winds at
850 hPa (NCEP/NCAR reanalysis) are performed to
identify the large-scale variability of the South America
monsoon system and the SACZ. The first EOFc was used
as a large-scale index for the South American monsoon
(LISAM), whereas the second EOFc characterized the
SACZ. LISAM (SACZ) index showed spectral variance on
30–90 (15–20) days and were both band filtered (10–
100 days). Intraseasonal wet anomalies were defined when
LISAM and SACZ anomalies were above the 75th per-
centile of their respective distribution. LISAM and SACZ
wet events were examined independently of each other and
when they occur simultaneously. LISAM wet events were
observed with the amplification of wave activity in the
Northern Hemisphere and the enhancement of northwest-
erly cross-equatorial moisture transport over tropical con-
tinental SA. Enhanced SACZ was observed with moisture
transport from the extratropics of the Southern Hemi-
sphere. Simultaneous LISAM and SACZ wet events are
associated with cross-equatorial moisture transport along
with moisture transport from Subtropical Southwestern
Atlantic.
1 Introduction
The South American monsoon system (SAMS) is charac-
terized by pronounced seasonality in the rainfall with the
wet season in the austral summer and a dry season in the
austral winter (Zhou and Lau 1998; Vera et al. 2006).
Unlike the Asian monsoon there is no reversal of the mean
surface wind. Nevertheless, the seasonal reversal of the
large-scale circulation over SA resembles those of a
monsoon system when the annual mean is removed (Zhou
and Lau 1998). Other monsoon like features are observed
such as the Bolivian High (Silva Dias et al. 1983), the
stretching of the of the atmosphere column and conversion
of low-level potential energy into upper-level divergent
kinetic energy, and the interaction between divergent and
rotational winds at the upper level that transform divergent
kinetic energy into rotational kinetic energy (Wang and Fu
2002). Other important features of SAMS are the Northeast
trough (Silva Dias et al. 1983; Lenters and Cook 1997), the
low level Gran-Chaco low (Gandu and Silva Dias 1998)
and the South Atlantic convergence zone (SACZ) (Kodama
1992).
The SACZ is the most significant feature of cloudiness
and precipitation in the wet season over eastern Tropical
L. M. V. Carvalho (&)
Department of Geography, University of California Santa
Barbara, Santa Barbara, CA 93106, USA
e-mail: [email protected]
L. M. V. Carvalho � A. E. Silva � P. L. Silva Dias � H. R. Rocha
Department of Atmospheric Sciences, University of Sao Paulo,
Sao Paulo, Brazil
L. M. V. Carvalho � C. Jones
Institute for Computational Earth System Sciences,
University of California Santa Barbara, Santa Barbara, CA, USA
B. Liebmann
CIRES Climate Diagnostics Center, Boulder, CO, USA
P. L. Silva Dias
National Laboratory for Scientific Computing, Petropolis, Brazil
123
Clim Dyn (2011) 36:1865–1880
DOI 10.1007/s00382-010-0806-2
South America and Southwestern Atlantic. It is charac-
terized by a northwest–southeast persistent cloud band
associated with low level convergence of winds and
moisture (Kodama 1992). Numerical modeling studies
have shown that the adiabatic heating in the Amazon
Basin during the summer monsoon induces a regional
tropospheric circulation that supports the NW/SE ori-
ented low level convergence that maintains the conti-
nental component of the SACZ (Figueroa et al. 1995;
Gandu and Silva Dias 1998). In addition, the SACZ is
characterized by large spatial and temporal variability
that depends on the interplay of phenomena on a broad
range of scales (Carvalho et al. 2002a, b). Moreover, the
intensification and weakening of the SACZ and its
extension toward subtropical southwest Atlantic Ocean is
strongly modulated by intraseasonal activity (Nogues-
Paegle and Mo 1997; Liebmann et al. 1999; Carvalho
et al. 2004; Muza et al. 2009). The variability of the
SACZ and its extent toward the continent and compen-
sating subsidence over southern Brazil, Northeast
Argentina and Uruguay (Nogues-Paegle and Mo 1997;
Liebmann et al. 1999; Gandu and Silva Dias 1998;
Figueroa et al. 1995) affect densely populated regions of
southeastern South America and important agricultural
poles and play a fundamental role in the hydrological
cycle in these regions (Liebmann et al. 2001; Carvalho
et al. 2002a).
The onset of intense convective activity and heavy
precipitation over most of the Amazon, central and
southeastern Brazil in the present climate is between
October and November; it peaks in the austral summer
(December–February) and the end of the rainy season over
central and southeast Brazil is between the end of March
and mid-April as intense precipitation gradually migrates
from the south Amazon and central Brazil toward the
equator (Kousky 1988; Horel et al. 1989; Marengo et al.
2001; Gan et al. 2004; Vera et al. 2006; Silva and Carvalho
2007; Bombardi and Carvalho 2009). Coupled atmo-
sphere–ocean and land interactions modulate variations of
SAMS on a broad range of timescales (Barreiro et al. 2002;
Grimm 2003; Chaves and Nobre 2004; De Almeida et al.
2007; Misra 2008).
An important characteristic of SAMS is the frequent and
persistent active and inactive (or ‘‘break’’) periods in the
intensity of rainfall. Tropical intraseasonal oscillations
have been found to strongly modulate these active and
break periods with impacts in circulation (Casarin and
Kousky 1986; Silva Dias and Marengo 1999; Jones
and Carvalho 2002), mesoscale activity (Carvalho et al.
2002b; Petersen et al. 2002), extreme precipitation
(Carvalho et al. 2004) and the seesaw of wet and dry
periods over eastern South America (Nogues-Paegle and
Mo 1997; Muza et al. 2009).
Changes in circulation caused by intraseasonal oscilla-
tions have important impacts on moisture transport and
water balance in the monsoon region. Herdies et al. (2002)
examined the moisture budget over several areas of tropical
and subtropical South America during times characterized
by the presence of the SACZ and times with no SACZ
(NSACZ) during the 1999 January–February TRMM-LBA
campaign (Avissar et al. 2002). They verified that during
SACZ (NSCZ) conditions strong (weak) convergence is
observed over the Amazon basin with divergence (con-
vergence) over southwestern Brazil, northern Argentina
and Paraguay. They showed that during the active phase of
the SACZ strong moisture flow from the Atlantic Ocean
entered South America and is funneled to southeastern
Brazil. These events are accompanied by strong conver-
gence in the Amazon basin, central eastern and south-
eastern Brazil.
The objectives of the present study are multiple. We
propose the use of a large-scale index for the South
America monsoon (LISAM; Silva and Carvalho 2007) to
characterize the spatial–temporal features of the SAMS.
Similar methodology will be used to identify the SACZ.
The characterization of the SACZ as an independent fea-
ture of the continental component of the SAMS helps in
understanding the mechanisms associated with moisture
transport and ultimately its variations over time. In addition
we will examine moisture transport during the occurrence
of intraseasonal oscillations that affect SAMS and the
SACZ. Our goal is to provide a large-scale analysis of the
moisture fluxes during wet regimes. This study is organized
as follows. Section 2 discusses datasets used in this study.
Section 3 proposes a methodology to distinguish the con-
tinental versus oceanic activity of SAMS. Section 4
examines separately continental and oceanic activity in
association with SAMS. Section 5 shows the importance of
wave activity in the southern and northern hemisphere in
modulating moisture fluxes and precipitation. Section 6
discusses the main conclusions.
2 Data
The present study examines intraseasonal variations in
SAMS and SACZ during the SH wet season (December–
February). This analysis extends from 1979 to 2007. The
datasets examined in this study are pentad (5-day average)
precipitation estimates from the Global Precipitation
Climatology Project (GPCP) (Xie et al. 2003), specific
humidity (q), air temperature (T), zonal (u) and meridional
(v) wind components, pressure (p) from the National
Centers for Environmental Prediction/National Center for
Atmospheric Research (NCEP/NCAR) (Kalnay et al. 1996)
with 2.5 latitude and longitude degrees resolution.
1866 L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability
123
3 Characterization of SAMS and SACZ
To characterize the monsoon regime a large-scale monsoon
index was obtained by the method discussed in Silva and
Carvalho (2007). To do so a combined EOF analysis
(EOFc) was performed using anomalies (climatological
annual mean removed) of the following variables: GPCP
estimates, q, v, u, and T (850 hPa). The first combined
EOFc mode (EOFc-1), which explains 24% of the total
variance, represents the seasonal covariability of the most
important variables that have been related to SAMS (Zhou
and Lau 1998; Gan et al. 2006). Silva and Carvalho (2007)
demonstrated that the EOFc-1 time coefficient can be used
as a large-scale South America Monsoon index (henceforth
LISAM) to identify the onset, demise, intensity and intra-
seasonal variation of SAMS. Moreover, seasonal (October–
March) spectral analysis of LISAM shows a peak on
intraseasonal time-scales between 30 and 90 days (Silva
and Carvalho 2007). This peak is consistent with the
Madden–Julian oscillation (Madden and Julian 1994), which
is known to play an important role in modulating convection
over eastern SA during the summer (Carvalho et al. 2004;
Jones et al. 2004a, b). Silva and Carvalho (2007) showed that
positive (negative) anomalies of LISAM are related to active
(break) phases of the monsoon and respective patterns of
enhanced (suppressed) precipitation and westerly (easterly)
low-level circulation intraseasonal anomalies (Carvalho
et al. 2002b, 2004; Jones and Carvalho 2002).
Correlations between LISAM and anomalies (annual
mean removed) of GPCP precipitation, and 850 hPa q, u, v
and T are shown in Fig. 1. The large scale enhancement of
precipitation extending from the intertropical convergence
zone (ITCZ) toward southeastern Brazil with maximum
signal over the central Amazon characterizes the main
precipitation patterns associated with SAMS (Fig. 1a). This
pattern is observed along with an enhancement of specific
humidity at 850 hPa over most continental SA (Fig. 1b),
westerly (easterly) low level winds over central (southern)
Brazil (Fig. 1c), northerly low level winds over northern
SA (Fig. 1d) and enhancement of low level temperature
over eastern Brazil, Atlantic Ocean and subtropics of SA
(Fig. 1e). Details on the EOFc analysis including tests of
sensitivity for the appropriate set of variables used in this
study are thoroughly discussed in Silva and Carvalho
(2007).
The second combined EOF (EOFc-2) is independent of
LISAM according to the North et al. (1982) criterion and
explains about 10% of the total variance. EOF2c is a year-
round pattern. Correlations between EOFc-2 and precipi-
tation, low-level circulation and temperature anomalies
(annual mean removed) during the summer (November–
February) are shown in Fig. 1 and are clearly distinct from
the respective patterns obtained with LISAM. For instance,
EOFc-2 shows high positive correlation with precipitation
(Fig. 1f) over the southwest tropical Atlantic and negative
correlation over southern Brazil and La Plata Basin. High
specific humidity in 850 hPa over southeastern Brazil
extending toward the Atlantic Ocean (Fig. 1g) is observed
along with low-level westerly winds over the same region
(Fig. 1h). Northerly (southerly) winds over the northern
(southern) flank of maximum precipitation (Fig. 1i) are
consistent with the presence of the SACZ (e.g., Kodama
1992). Moreover, negative temperature anomalies (Fig. 1j)
are approximately collocated with southerly winds over the
southern flank of the SACZ. These features have been
previously identified as the seesaw pattern of convective
activity and circulation associated with the organization of
the SACZ (e.g., Nogues-Paegle and Mo 1997; Liebmann
et al. 1999; Herdies et al. 2002; Carvalho et al. 2004). More
importantly, EOFc-2 is consistent with previous observa-
tions that convection over the southwest South Atlantic
typically associated with the SACZ can be decoupled from
convective activity over continental SA during summer
(Carvalho et al. 2002b, 2004; Muza et al. 2009). EOF2-c
time coefficient will be used as an index to identify the
oceanic SACZ (Carvalho et al. 2002b, 2004) and will
hereafter be referred to as the SACZ index.
Figure 2 shows an example of the time variability of
SACZ and LISAM indexes from 2003 to 2007. LISAM
shows a clear annual cycle with superimposed high fre-
quency fluctuations. The onset (end) of the rainy season
was defined in Silva and Carvalho (2007) when the three-
pentad (15 days) moving average of LISAM becomes
positive (negative). The EOFc-2 time-coefficient, on the
other hand, is clearly associated with high frequency
variability during all seasons. It is worth emphasizing that
the EOFc-2 pattern, here referred to as the oceanic SACZ
mode, is not strictly a summer feature, as indicated in
Fig. 2. The spatial patterns of correlation obtained between
EOFc and above mentioned variables are typical of oceanic
cold fronts as indicated in Fig. 1g and j (moisture and
temperature, respectively). These synoptic patterns have
been previously identified by Kousky (1979) and Kousky
and Ferreira (1981) as present both in winter and summer.
Spectral analysis of the SACZ index during the wet
season (November–March) indicates a statistically signifi-
cant peak in the spectrum approximately between 15 and
20 days (not shown), consistent with previous observations
(Liebmann et al. 1999; Nogues-Paegle et al. 2000).
Extratropical Rossby wave trains are the main forcing for
the intraseasonal variability of the SACZ (Casarin and
Kousky 1986; Grimm and Silva Dias 1995; Nogues-Paegle
and Mo 1997; Liebmann et al. 1999; Jones and Carvalho
2002; Carvalho et al. 2004). Some of these wave trains
have been shown to be associated with the eastward
propagation of the MJO (Liebmann et al. 2004). LISAM
L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability 1867
123
Fig. 1 Correlation between
LISAM and anomalies (annual
mean removed) of precipitation
(a), specific humidity (b), zonal
wind (c), meridional wind (d),
air temperature (e) in 850 hPa.
Correlations between SACZ and
precipitation (f), specific
humidity (g), zonal wind (h),
meridional wind (i), air
temperature (j) in 850 hPa
1868 L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability
123
represents large-scale variations of the monsoon over
tropical SA and its spectral peak on longer time-scales (30–
90 days) is consistent with the high variance of Kelvin
waves observed in this region during the austral summer
(Liebmann et al. 2009).
4 Continental versus oceanic intraseasonal activity
The identification of the continental and oceanic variability
of convective activity as two distinct modes of SAMS
variability is of great relevance for the atmospheric com-
ponent of the regional hydrological cycle. For instance,
previous studies (Carvalho et al. 2002a) have shown that
coastal stations over southeastern Brazil are more affected
by extreme precipitation during El Nino years than inland
stations. Excessive rainfall along the coastal area of
Southern and Southeastern Brazil has been associated with
low level winds perpendicular to the mountain range that
runs parallel to the coast (Xavier and Silva Dias 1995).The
proximity to the coast implies therefore in a larger influ-
ence of the oceanic activity of the SACZ which can be
independent of the rainfall regime over continental areas.
Muza et al. (2009) showed that distinct dynamical mech-
anisms influence the continental and oceanic convective
activity associated with the SACZ on interannual time-
scales.
In this study we objectively separate oceanic from
continental convective activity using LISAM and SACZ
indexes. Our goal is to understand moisture fluxes during
anomalous wet events on intraseasonal time-scales when
the monsoon is well established (December–February). For
this purpose LISAM and SACZ indexes were both band
filtered on intraseasonal timescales (10–100 days) using a
band-pass filter in frequency domain. Maximum correla-
tion between the two filtered indexes is observed at zero lag
(*0.24) and is not statistically significant at 5% signifi-
cance level. No statistically significant correlation was
obtained for lags varying from -6 to ?6 pentads.
Intraseasonal wet anomalies were then defined when
LISAM and SACZ anomalies were above the 75th per-
centile of the distribution of the respective anomalies
(hereafter referred to as LISAMIS and SACZIS). Although
the two indexes are uncorrelated in time, there are some
events during which LISAMIS and SACZIS simultaneously
satisfy the wet event criterion. These events are potentially
relevant as they indicate a simultaneous enhancement of
convection over southeastern SA and the subtropical
Atlantic Ocean. We examined separately all LISAMIS and
SACZIS events that were not coincident, that is they did not
simultaneously satisfy the wet LISAMIS and wet SACZIS
criteria. Likewise, we also investigate those events during
which LISAMIS and SACZIS simultaneously satisfied the
wet criteria for at least one pentad (henceforth referred to
as LISAMIS ? SACZIS events).
Thirty LISAMIS, thirty nine SACZIS and thirty eight
LISAMIS ? SACZIS independent events were examined in
this study during DJF of the 1979/1980–2006/2007 mon-
soon seasons. Independent events are considered as events
that are at least two pentads apart. Figure 3 shows com-
posites of outgoing longwave radiation (OLR) intrasea-
sonal anomalies (10–100 days) for LISAMIS, SACZIS and
LISAMIS ? SACZIS events. OLR anomalies were used in
this analysis as an independent proxy for convective
Fig. 2 Example of LISAM and
SACZ indexes during
2003–2007
L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability 1869
123
activity that was not used in the definition of LISAM and
SACZ indexes. Composites are shown only for lag 0. They
clearly indicate that LISAMIS events are associated with
intense convective activity over tropical central-eastern
Brazil and suppression over northern SA (Fig. 3a). The
existence of a dipole in the convective activity on intra-
seasonal time-scales between northern and central Amazon
has been previously identified in (Carvalho et al. 2002a). In
contrast, SACZIS wet events (Fig. 3b) are associated with
convective activity displaced toward the Atlantic Ocean
along with suppression of convection over southern Brazil,
Uruguay, southern Argentina, and in the ITCZ region,
which indicates association with wave trains. There is
also indication of enhancement of convective activity
over northwestern SA during these events (Fig. 3b).
LISAMIS ? SACZIS events (Fig. 3c) clearly indicate the
combined enhancement of convection over the continent
with extension toward the subtropical Atlantic Ocean
associated with simultaneous wet LISAMIS and SACZIS
anomalies. Suppression of convective activity is observed
over southern Brazil, Uruguay and northeastern Argentina,
which characterize the seesaw in convective anomalies on
intraseasonal time scale that has been extensively docu-
mented in previous papers (Nogues-Paegle and Mo 1997;
Liebmann et al. 1999; Carvalho et al. 2004). Lag-com-
posites of OLR intraseasonal anomalies do not clearly
indicate coherent patterns of convection over the tropical
and equatorial Pacific that resemble the canonical propa-
gation of MJO and its remote influence on SA, as sug-
gested in previous studies (Carvalho et al. 2004; Jones
et al. 2004a). It is important to note, however, that there is
a large case-to-case variability in the convective activity
related to the propagation of the MJO (Wang and Rui
1990; Jones et al. 2004b) and links with the oscillation
based on composites of OLR intraseasonal anomalies can
be misleading.
Fig. 3 OLR Intraseasonal
anomalies (10–90 days) for
LISAMIS (a) SACZIS (b) and
LISAMIS ? SACZIS (c)
1870 L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability
123
Figure 4 shows the interannual distribution of wet
LISAMIS and SACZIS events during the 1979–2007 mon-
soon seasons. At least one LISAMIS and/or SACZIS wet
event occurred every season, with exception of the 1981/
1982, 1982/1983, 1989/1990 and 2006/2007 DJF seasons.
LISAMIS ? SACZIS events dominate the wet events dur-
ing these years. In the 1979/1980–1988/1989 decade the
total number of LISAMIS and SACZIS wet events is equal 8
and 10, respectively. In the 1990/1991–1999/2000 decade
the number of LISAMIS and SACZIS wet events approxi-
mately doubled to 17 and 19 events, respectively. No sig-
nificant differences are observed for LISAMIS ? SACZIS
events, which varied from 15 in the 1979–1989 decade to
11 in the 1990–2000 decade. The total number of wet
events classified in Fig. 4 is equal 33 in the 1979–1989
decade against 47 in the 1990–2000 decade, which repre-
sents an increase of 42% in the latter period. Jones and
Carvalho (2006, 2009) have shown evidence of decadal
variability in the number of MJO events with the 1990s
more active than the 1980s.
5 Moisture transport and intraseasonal activity
The propagation of tropical and extratropical intraseasonal
disturbances causes significant changes in regional atmo-
spheric circulations (Jones and Carvalho 2002; Carvalho
et al 2002a; Liebmann et al. 2004) and in moisture trans-
port (Herdies et al. 2002) over SA that can persist from one
up to five pentads (Muza et al. 2009). The objective of the
present study is to evaluate the transport of when convec-
tion increases over central SA during active phases of the
monsoon with no evident organization of the SACZ over
the ocean (LISAMIS events). Likewise, we will investigate
how moisture transport changes when the SACZ is active
along with weak convective activity over the continent
(SACZIS events) and when convection over southeastern
SA and southwestern Atlantic is very intense (LISAMIS ?
SACZIS events). The spatial resolution of the grid cells
considered in this study is 2.5�92.5� latitude/longitude
(*105 km2 in the tropics).
The vertically integrated moisture flux (kg m-1 day-1)
was decomposed into zonal fu and meridional fv compo-
nents according to Peixoto and Oort (1992) based on the
following approach:
fu ¼ 1
g
Zn
nþ1
qudp ¼ 1
g
XN
n¼1
uqð Þnþ1þ uqð Þn� �
2pn � pnþ1½ � ð1Þ
fv ¼ 1
g
Zn
nþ1
qvdp ¼ 1
g
XN
n¼1
qvð Þnþ1þ qvð Þn� �
2pn � pnþ1½ � ð2Þ
where q is the specific humidity (kg kg-1), u and v are the
wind zonal and meridional components, respectively
(m s-1). The integral is calculated from pressure p at sur-
face up to 300-hPa.
In this study we examine variations in the moisture
transport associated with LISAMIS, SACZIS and
LISAMIS ? SACZIS wet events. Moisture transport will be
examined by computing the zonal (Eq. 1) and meridional
(Eq. 2) components of the integrated moisture flux during
the DJF seasons. Anomalies with respect to the climatology
were computed for both components separately and then
summed. The description of the dynamical systems
observed during the events will be addressed by lag-com-
posites of streamfunction anomalies.
5.1 LISAMIS wet events
Lag composites of vertically integrated moisture flux
anomalies during LISAMIS wet events are shown in Fig. 5.
About one pentad before the occurrence of LISAMIS wet
events the anomalous integrated moisture flux indicates the
amplification of a trough in the Northern Hemisphere over
the subtropical Pacific near the west coast of North
America (Fig. 5a). This trough transports moisture from
the tropics and subtropics of the western Pacific toward
California. The wave like pattern is also evident from
midwestern to southeastern North America and the Carib-
bean and indicates an increase in the equatorward transport
of humidity in this region. Northerly moisture transport is
observed over northern SA at the same lag. There is also
indication of wave activity in the extratropics and sub-
tropics of the Southern Hemisphere as well, which
increases the easterly contribution of the vertically inte-
grated moisture flux over southeastern Brazil. Concomi-
tantly with LISAMIS wet events (Fig. 5b) there is an
intensification of the northwesterly moisture flux across the
equator toward tropical SA. Moisture fluxes are larger overFig. 4 Interannual variability in the number of LISAMIS, SACZIS
and LISAMIS ? SACZIS wet events
L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability 1871
123
central-eastern SA. One pentad later (Fig. 5c) low level
northwesterly anomalies near equatorial SA still persist
(not shown) and maintain the westerly moisture fluxes over
northeastern SA approximately between 10�S and 10�N.
Two pentads after LISAMIS wet events there is no con-
sistent anomaly of moisture fluxes over SA (not shown).
Rossby waves propagating into the tropics have been
identified in association with westerly flow that extends
from the extratropics into low latitudes (Kasahara and Silva
Dias 1986; Hsu and Lin 1992; Kiladis and Weickmann
1992a, b; Tomas and Webster 1994). Strong westerly jets
such as the Asian and the Southern Hemisphere jets can act
as Rossby waveguides with wave paths arching across
North America and SA (Hoskins and Ambrizzi 1993).
These westerly jets have a fundamental role in the energy
conversion from the baroclinic perturbations driven by
latent heat release in the tropics into barotropic anomalies
that characterize the teleconnection patterns (Kasahara and
Silva Dias 1986).
Rossby wave activity in both hemispheres in association
with LISAMIS wet events can be observed from lag com-
posites of 200-hPa streamfunction anomalies (Fig. 6).
Notice that no filtering was applied to these fields in order
to keep most of the high frequency variability of the
midlatitude Rossby waves (Kiladis and Weickmann 1992a,
b). A well characterized wave-like pattern in 200 hPa
streamfunction anomalies is observed extending from the
Equator toward high latitudes of Northern Hemisphere one
pentad preceding LISAMIS wet events (Fig. 6a). This
feature follows the Pacific-Atlantic wave path shown in
Hoskins and Ambrizzi (1993) and resembles the Pacific
North American (PNA) teleconnection pattern (Wallace
Fig. 5 LISAMIS lag-
composites of the integrated
moisture flux
1872 L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability
123
and Gutzler 1981) in its positive phase. The PNA pattern is
coupled to fluctuations in the strength and location of the
Asian-jet stream and to anomalous convection in
the equatorial Pacific (Grimm and Silva Dias 1995). The
positive phase of the PNA indicates an enhancement of the
East Asian jet stream and an eastward shift in the jet exit
region toward the western United States and increased
upward motion over the Central Equatorial Pacific and
midtropospheric subsidence in the region of the upper
westerly jet.
The positive phase of the PNA is related to the ano-
malous vertically integrated moisture transport by the two
pair of troughs over western and eastern North America
(Fig. 5a). In the Southern Hemisphere there is indication of
an intensification of the upper cyclonic vortex off the
Northeast coast of Brazil, known as the Northeast trough
(Lenters and Cook 1997; Gandu and Silva Dias 1998)
along with a pair of anti-cyclones in mid latitudes (Fig. 6a).
The latter are related to the weakening of the subtropical jet
near the southern coast of Brazil and La Plata basin (not
shown).
Concomitantly with LISAMIS wet events the PNA
remains in its positive phase and seems to intensify over
the tropical Pacific and Atlantic (Fig. 6b). Concurrently,
low level westerly anomalies are observed near the equator
(not shown) along with a cross equatorial transport of
moisture, intensification of moisture fluxes over central
Brazil (Fig. 5b) and enhanced convection in the same
region (Fig. 3a). Early studies have shown that cross
equatorial dispersion of wave energy from the extratropics
of the Northern Hemisphere toward the Southern Hemi-
sphere increases with the magnitude of the westerlies over
the equator (e.g., Webster and Holton 1982). The Southern
Hemisphere connection with the PNA has been attributed
to the anomalous heat source in the equatorial Pacific
(Grimm and Silva Dias 1995). The anomalous heat source
Fig. 6 200 hPa streamfunction
anomalies (climatology
removed) during LISAMIS
wet events
L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability 1873
123
in the central and western Equatorial Pacific triggers
Kelvin waves that generate strong upper level westerly
flow anomalies in the eastern Pacific. In this study, the
cross-equatorial anomalous flux is not clearly observed in
association with convective activity along the Equatorial
Pacific from OLR anomalies, possibly due to large case-to-
case variability. The intensification of moisture fluxes north
of the Equator one pentad before LISAMIS wet (Fig. 6a)
seems to contribute to the intensification of convective
activity near Equatorial South America. Convectively
coupled Kelvin waves have non negligible meridional
component in regions of deep convection (Straub and
Kiladis 2002; Dias and Pauluis 2009). The upper tropo-
spheric Kelvin wave, with the associated baroclinic struc-
ture, is revealed in the intensification of the zonal moisture
flow in the equatorial portion of the South American
continent (Fig. 5b).
As convection intensifies over the Amazon and central
Brazil, the Bolivian high becomes well established (Silva
Dias et al. 1983) along with the Northeast trough (Gandu
and Silva Dias 1998). Westerly wind anomalies in 200 hPa
intensify near the Equator off the west coast of SA toward
central eastern SA where most of the convective activity
takes place (not shown). One pentad after the LISAMIS wet
events the PNA pattern over north Pacific and the
East-Asian jet weaken, although the PNA remains in its
positive phase (Fig. 6c). The weakening of the PNA is
observed along with the decrease in the vertically inte-
grated moisture flux across northern SA (Fig. 5c). Mean-
while, the Northeast trough weakens and upper level
positive vorticity dominates in the midlatitudes of the
Southern Hemisphere. These combined features charac-
terize the weakening of LISAMIS wet events.
5.2 SACZIS wet events
Lag composites of the anomalies of the vertically inte-
grated moisture flux during SACZIS wet events are shown
in Fig. 7. One pentad before the events (Fig. 7a) there is
evidence of a wave train in the extratropics of the Southern
Hemisphere that is transporting moisture from midlatitudes
toward southern Argentina. No statistically significant
anomalies are observed in the Northern Hemisphere prior
and during the event. For lag 0 (Fig. 7b) the easterly
moisture transport from subtropical Atlantic intensifies as
the wave train moves northward. Easterly anomalies in
moisture transport are observed over southeastern South
America at the southern flank of the SACZ. Northwesterly
anomalies are observed co-located with and at the northern
flank of the oceanic SACZ (compare with Fig. 3b). One
pentad after the event, anomalous convection weakens over
the ocean and the SACZ is no longer evident (not shown).
The weakening of the SACZ is observed along with the
intensification of the easterly moisture transport over
northwest SA (Fig. 7c).
Rossby wave activity in the Southern Hemisphere dur-
ing SACZIS wet events can be identified from lag com-
posites of anomalous streamfunction at 200 hPa (Fig. 8).
For consistency with previous analysis (Fig. 6) only the
annual cycle was removed and no further filtering was
applied. One pentad before SACZIS wet events (Fig. 8a) a
wave like pattern extending from Indonesia toward high
latitudes of the Southern Hemisphere becomes evident and
resembles the Pacific South American (PSA) teleconnec-
tion pattern (Grimm and Silva Dias 1995; Mo and Paegle
2001). The existence of a wave number-3 pattern becomes
much more evident in composites of 10–100 day band-
filtered streamfunction anomalies (not shown). Another
region with anti-cyclonic circulation is observed in the
subtropics of the Northern Hemisphere over central Pacific
but there is no evidence of the PNA pattern, as observed
during LISAMIS wet events (compare with Fig. 6a). No
statistically significant anomaly in the zonal winds in
200 hPa in association with Asian subtropical Jet is
observed one pentad prior, during or after the event. This is
a consequence of the connection between the Southern
Hemisphere wave train and the anomalous convection in
the SPCZ region that more strongly interacts with the
Southern Hemisphere summer teleconnection patterns
(Grimm and Silva Dias 1995). The PSA becomes well
characterized in high latitudes of the Southern Hemisphere
for lag 0 (Fig. 8b), consistent with the intensification of the
SACZ over the ocean (Fig. 3b). One pentad later the PSA
pattern weakens in the subtropics and extratropics of the
Southern Hemisphere and enhanced convection in the
SACZ is no longer evident (not shown).
5.3 LISAMIS ? SACZIS wet events
Lag composites during LISAMIS ? SACZIS wet events
were obtained by averaging only pentads when both
indexes satisfied the criterion of wet events discussed
before. It is possible, however, that in some cases SACZIS
or LISAMIS alone satisfied the criterion for a wet event
longer than the period when both indexes were above the
75th percentile. Those pentads were not included in the
composites. The vertically integrated moisture flux is
shown in Fig. 9. These events combine features discussed
previously for LISAMIS and SACZIS wet events and are
associated with the largest magnitude of the vertically
integrated moisture transport anomalies over the tropics.
One pentad before the event (Fig. 9a) westerly anomalous
moisture transport is observed at the subtropics of the
Northern Hemisphere over central Pacific and Atlantic.
These anomalies indicate the enhancement of zonal winds
in the lower and mid troposphere at these latitudes.
1874 L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability
123
Westerly moisture transport increases in the Southern
Hemisphere over tropical Pacific near the date line. There
is also evidence of wave activity in the Southern Hemi-
sphere indicated by the anomalous cyclonic transport in the
extratropics and subtropics of South America. For lag 0 the
anomalous cyclonic transport with center over subtropical
western Atlantic intensifies over the continent and the
ocean (Fig. 9b). There is indication of cross-equatorial
anomalous northwesterly transport of moisture extending
from the Amazon toward tropical Atlantic (Fig. 9b). In the
Northern Hemisphere, anomalous cyclonic moisture flux
over central subtropical Pacific persists for lag 0. One
pentad after the event (Fig. 9c), westerly anomalies are
observed over tropical South America and Equatorial
Pacific off the west coast of the continent and along the
Atlantic ITCZ (Fig. 9c). The anomalous cyclonic moisture
transport over subtropical Atlantic weakens whereas the
anomalous cyclonic moisture transport over subtropical
North Pacific intensifies (Fig. 9c).
Figure 10 shows lag composites of the streamfunction
anomalies during LISAMIS ? SACZIS wet events. One
pentad before the event (Fig. 10a), negative streamfunction
anomalies in high latitudes of the Northern Pacific and
positive over Tropical Pacific near the date line indicates
the enhancement of the boreal winter subtropical Asian jet.
Nevertheless, the PNA is not clearly observed as a wave
number-3 pattern from the tropics to the extratropics of the
Northern Hemisphere. The pair of cyclonic anomalies off
the west coast of North America and over eastern extra-
tropical North America and North Atlantic characterizes a
Fig. 7 The same as Fig. 5 but
for the SACZIS
L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability 1875
123
wave number-1 pattern in the streamfunction anomalies in
midlatitudes of the Northern Hemisphere. In the Southern
Hemisphere, however, the PSA becomes evident (Fig. 10a)
as a wave-3 pattern from Indonesia toward midlatitudes.
The Northeast trough is very pronounced one pentad prior
the event as part of the wave-1 tropical cyclonic anomaly
observed in low latitudes of the southern Hemisphere
(Fig. 10a). For lag 0 (Fig. 10b), a midlatitude wave train
that arches around SA intensifies in the extratropics and
subtropics which explains the enhancement of moisture
fluxes from the subtropical Atlantic Ocean observed
simultaneously with the event (Fig. 9b). The inter-hemi-
spheric transport of moisture indicated in Fig. 9b is con-
sistent with positive streamfunction anomalies north of the
Equator over the Caribbean and north of South America
(Fig. 10b). One pentad after the event (Fig. 10c), the wave
pattern over eastern SA weakens which is consistent with
the weakening of the LISAMIS ? SACZIS wet events.
6 Conclusions
This study proposes a new approach to investigate the
large-scale variability of SAMS by performing a combined
EOF of precipitation, temperature, moisture and circulation
anomalies (annual mean removed) over tropical and sub-
tropical South America. The first combined EOF (LISAM)
characterizes the monsoon as a seasonal reversal in pre-
cipitation and circulation anomalies over central South
America. The second combined EOF distinguishes the
oceanic activity that affects Southeastern Brazil with
opposite signal over southern Brazil, Uruguay and north-
eastern Argentina that has been associated with the SACZ.
The advantage of this approach is that it allows charac-
terizing the main large-scale features of SAMS without
restricting our analysis to boxes or any arbitrary region.
Moreover, multiple datasets such as NCEP/NCAR reana-
lysis and GPCP pentad precipitation are employed in this
Fig. 8 The same as Fig. 6 but
for SACZIS wet events
1876 L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability
123
analysis. Our objective in using the latter was to extend our
analysis to the Atlantic Ocean and therefore to characterize
the variability of the SACZ. The disadvantage of using
pentad data is to smooth out variations on synoptic scales
(period less than 10 days) which might account for large
variance of the spectrum in the monsoon region particu-
larly upon the SACZ. Nevertheless, LISAM and SACZ
indexes show spectral variance on intraseasonal time-
scales that have been extensively documented in previous
studies (e.g. Liebmann et al. 1999, 2009; Nogues-Paegle
and Mo 1997; Muza et al. 2009). These properties were
further used to investigate moisture transport during wet
events on intraseasonal timescales.
Enhanced amplitudes of SAMS continental activity on
intraseasonal timescales as determined with LISAM are
associated with cross-equatorial northwesterly moisture
flux anomalies over the Amazon and central Brazil. Cross-
equatorial flow over western Amazon has been related to
seasonal variations in precipitation over South America
(Wang and Fu 2002). Here we show that similar mecha-
nism drives variations on shorter timescales and are asso-
ciated with the strengthening of the Asian subtropical jet
and the PNA in its positive phase. These findings clearly
indicate that the cross-equatorial flux plays an important
role for changes in moisture fluxes that ultimately enhance
convection over central-eastern Brazil on intraseasonal
timescales. Those events represent an increase in the con-
tribution to the monsoon precipitation from moisture
transported from northern Amazon toward eastern South
America
In contrast, enhanced amplitudes of convective activity
on intraseasonal timescales identified by the SACZ index
Fig. 9 The same as Fig. 5 but
for LISAMIS ? SACZIS wet
events
L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability 1877
123
are related to high wave activity in the Southern Hemisphere
in association with the PSA. In these cases, there is no
evidence of enhancement in the Asian subtropical jet and of
the PNA pattern. No cross-hemispheric transport of mois-
ture is evident. Moisture from the Atlantic Ocean is trans-
ported by wind anomalies associated with wave trains.
Enhanced convection over eastern Brazil extending
toward the Atlantic Ocean has been often observed (e.g.
Marton and Silva Dias 2001; Carvalho et al. 2004) and was
identified here from combined positive anomalies in the
band-filtered LISAM and SACZ indexes. These events
occur when wave activity in the Southern Hemisphere is
well characterized along with the strengthening of the
subtropical jet in the Northern Hemisphere. Cross-equato-
rial transport of humidity occurs in combination with the
intensification of the transport from the western tropical
Atlantic Ocean forced by anomalous cyclonic activity near
the coast associated with the propagation of midlatitude
wave trains. These events indicate the importance of both
sources of humidity, the Amazon and the Atlantic Ocean
for the enhancement of convection over eastern South
America.
Acknowledgments This research was funded by NOAA Office of
Global Programs (NOAA/NA05OAR4311129, NA07OAR4310211
and NA08OAR4310698). NCEP/NCAR Reanalysis data provided
by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from
their Web site at http://www.cdc.noaa.gov. Dr. Ana E. Silva
thanks funding from FAPESP and CAPES. Dr. Leila M. V. Carv-
alho and Ana Silva thank funds from CNPq and CLARIS
LPB project. Dr. Pedro L. Silva Dias acknowledges the CNPq
support.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
Fig. 10 The same as Fig. 6 but
for LISAMIS ? SACZIS wet
events
1878 L. M. V. Carvalho et al.: Moisture transport and intraseasonal variability
123
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